1. Technical Field
This invention relates to telephony in general, and in particular, to a telephone set for hearing impaired users that provides full duplex operation while supplying at least 50 dB of gain to a received signal without inducing unstable operation, or “feedback howl,” in the set.
2. Related Art
A graphical representation of the “auditory area” of a young, adult subject with normal, average hearing is shown in FIG. 1 in terms of the frequency and intensity ranges of sounds that are audible to the subject. As shown in the figure, the auditory area is bounded, in terms of audible frequencies, between about 20 Hz and 18 kHz.
In terms of intensity, as measured in decibels (“dB”) relative to a reference sound pressure level (“SPL”) of 20 micro-Pascals (20 μPa), the auditory area is bounded between two roughly upwardly-concave curves. The lower of these demarcates the “threshold of hearing” level, below which sounds are inaudible, and the upper curve demarcates the “threshold of feeling” level, at which sounds begin to produce a tickling sensation in the ear. Sounds with intensities above the threshold of feeling can cause pain and permanent damage to the ear. For this reason, Underwriters Laboratories, Inc., has promulgated a safety standard for telephones (UL 1950, 3rd ed.) that fixes an upper limit of 124 dB SPL on the intensity of sound that the earphone, or receiver, of a UL-certifiable telephone can produce, as shown in FIG. 1.
Subsumed within the auditory area of FIG. 1 are two overlapping regions of interest in the field of sound reproduction. The first of these is the area within which all sounds normally associated with the psycho-acoustic phenomenon of music occur, and the second, which is subsumed within the first, is the area within which all sounds normally associated with the psycho-acoustic phenomenon of speech occur.
It may be seen that the speech auditory area is considerably smaller than the music auditory area, and extends from about 100 Hz to about 6 kHz in frequency, and from about 25 dB to about 70 dB SPL in intensity. Moreover, it has been found that good speech intelligibility can be obtained with a minimal loss in sound quality, and with a substantial savings in transmission bandwidth, by limiting the telephonic bandwidth, or “pass band,” to about 3 kHz, with frequencies ranging from between about 200 Hz to 3.2 kHz. Thus, all telephonic speech sounds can be thought of as occurring within the rectangular “pass window” outlined FIG. 1 between 200 Hz and 3.2 kHz on the frequency axis, and between about 25 dB and 70 dB SPL on the intensity, or “volume” axis.
All persons suffer some deterioration in the acuity of their hearing with age, men more so than women, as is illustrated in FIGS. 3 and 4, respectively, where the average deterioration in hearing in decibels at given frequencies with age in years are respectively plotted for men and women subjects. As may be generally inferred from FIGS. 3 and 4, more than 80% of individuals who experience a loss in hearing suffer from a loss in the high frequency range, which, for telephonic hearing, is defined to extend from about 1.8 to 3.2 kHz. Hearing losses may also be caused by other factors not related to age, including heredity, injury, and disease.
The telephonic speech pass window of FIG. 1 is reproduced in FIG. 2, which illustrates the effect of a “flat,” 60 dB hearing loss on telephonic communications. It may be seen that such a loss reduces a normal dynamic range of hearing DR1 of about 124 dB to a residual dynamic range DR2 of about 64 dB, thereby effectively “masking” a substantial portion of the telephonic speech pass window (shown by the solid outline), which has a dynamic range DRp of about 45–50 dB. Thus, nearly 80% of the telephonic speech received by an individual with such a hearing loss is inaudible to the individual, and therefore, lost. Accordingly, one of the principal objects of any hearing-assistive device, such as a hearing aid or hearing assistive telephone, is to shift the received speech program window upwards such that its dynamic range DRp falls within the remaining dynamic range DR2 of a hearing-impaired listener, as shown by the dashed outline in FIG. 2. In so doing, it may also be necessary to reshape the window, as with compression and limiting, such that it fits within that range.
The above object can be partially achieved in an amplified telephone set, such as the prior art amplified telephone set 10 illustrated schematically in FIG. 5 and discussed in more detail below, by simply adding linear gain, or amplification, to the received signal. However, because of unpredictable levels of feedback in the system, including “line echo,” “side tone,” and “acoustic echo,” there are at least two substantial limitations to this solution: First, since the telephone set forms part of a closed loop, not more than about 30 dB of gain can be added to the received signal without exceeding a total closed loop gain of unity, thereby inducing unstable operation, or “howl,” in the telephone, that is similar to that set up in a public address (“PA”) system when the microphone is placed too close to the output of the speaker. However, as may be seen from FIG. 2, as much as 45–50 dB of gain may be needed to completely shift the received signal's pass window into a hearing impaired user's remaining dynamic range of hearing, leaving a substantial deficit in the required amplification of 15–20 dB.
Second, the level of the hearing impaired user's transmitted signal that is fed back to the user in the form of line echo and side tone varies greatly and seldom matches that of the signal received by the user from a far-end talker. If the user adjusts the amplification of the set such that one of these signals is heard comfortably, the other signal may be either inaudibly soft, i.e., not heard, or heard uncomfortably loud and distorted. While these drawbacks can be addressed to a certain extent by so-called “echo suppression” techniques, i.e., switching circuits that prevent the user from transmitting and receiving at the same time, this not only can add “switching artifacts” to the conversation, i.e., annoying pops and clicks, but it also reduces the hearing-impaired user's communication from “full-duplex,” in which he can both transmit and receive simultaneously, to “half-duplex,” in which all side tone is effectively eliminated. Since side tone provides important telephonic and voice modulation cues to the speaker, hearing-impaired persons need side tone as much or more than persons with normal hearing for effective communication.
Accordingly, a long felt and as yet unsatisfied need exists for a telephone set for hearing impaired users that affords the user with effective full duplex communication, and that enables as much 50 dB of gain to be applied to the received signal without inducing feedback howl in the set.